Control of composition of overhead vaporous product in a partially condensing fractionation column

ABSTRACT

A multicomponent feed is fractionated in a distillation column to produce an overhead vapor stream, which is then partially condensed and passed to an accumulator to obtain a liquid external reflux stream and an overhead vaporous product stream. The accumulator pressure is maintained substantially constant. A signal representative of the predicted accumulator temperature is established, based on measurements of feed flow rate, feed composition, overhead product specifications, accumulator pressure, and bottoms product flow rate. The predicted accumulator temperature is compared with the measured accumulator temperature to obtain a bias signal. This bias signal is used to manipulate the bottoms product flow rate to effect the adjustment of the accumulator temperature to a value at which the concentration of a key component of the overhead vaporous product is substantially equal to the desired concentration thereof.

United States Patent [72] Inventors JimB.Palmer 3,296,097 l/l967 Lupfer3,322,650 5/1967 Hillburn.

3,338,825 8/1967 Taggart 3,449,2 l 5 6/1969 Johnson et al.

Primary Examiner-Wilbur L. Bascomb, Jr Attorney-Young and Quigg [54]CONTROL OF COMPOSITION OF OVERHEAD VAPOROUS PRODUCT IN A PAR-"ALLYABSTRACT: A multicomponent feed IS fractionated in a CONDENSINGFRACTIONATION COLUMN distillation column to produce an overhead vaporstream, 10 Claims, 2 Drawlng Figs.

which is then partially condensed and passed to an accumulator to obtaina liquid external reflux stream and an overhead vaporous product stream.The accumulator pressure is main- 203/ l, 203/2, 203/3, 62/37, 203/D1G.l tained substantially constant. A signal representative of the 235/196/141 predicted accumulator temperature is established, based on [51]Int. Bold 3/42 measurements of feed flow rate feed composition overhead[50] FieldolSearch..................,....................... 203/2, 1,3;product specifications accumulator pressure, and bottoms 203/DIG. 18;235/151 12 202/160; 196/132 141; 62/21 37; product flow rate. Thepredicted accumulator temperature is compared with the measuredaccumulator temperature to obtain a bias signal. This bias signal isused to manipulate the bottoms product flow rate to effect theadjustment of the accumulator temperature to a value at which theconcentration [56] References Cited UNITED STATES PATENTS 196/141 of akey component of the overhead vaporous product is sub- 203/2 stantiallyequal to the desired concentration thereofi 3,039,94l 6/l962 Sweeney3,294,648 12/1966 Lupferetal..................

l l I l l I l l l l l l l l ll .w 2 4 5 6 i m m In an mm |||i|| m CM I dm 5 i6 8 .3 L 4 u 3 A 3 w I 4 4 5 Y8 hi I war/ W M n an w m 1 a 6 5 5 5l l II M A 1 l l l l l l l A 7% H n 8 Q 2 M R S W 7 R e V 5 m w b a manllL M 2 U D wwe H UMW WH W he. o WM w i 5 l A w a F c rl 1 l l i IIIIWvI|\J.|||T RVJ F2 l l l l I l l l l I l lLrllwL 5% l I l ||Lll1llL CONTROLOF COMPOSITION OF OVERHEAD VAPOROUS PRODUCT IN A PARTIALLY CONDENSINGFRACTIONATION COLUMN This invention relates to apparatus for controllinga fractional distillation column, wherein only a part of the overheadfrom the column is condensed.

D.E. Lupfer has described in US. Pat. 3,296,097, issued Jan. 3, 1967, asystem for predictively controlling a fractional distillation column.While this system has proved advantagepus in various fractionationsystems, it has become desirable to provide an improved control systemfor a fractional distillation system wherein only a part of the vaporousoverhead stream from the column is condensed, the remaining vapor beingwithdrawn as a vaporous overhead product stream. When no vapor isremoved from the accumulator in a totally condensing system, thecompositions of the liquid overhead product stream and the liquidexternal reflux stream are the same as the composition of the vaporousstream withdrawn from the column overhead. However, when only a part ofthe vaporous overhead stream is condensed, the compositions of thevaporous overhead product and of the liquid external reflux can differfrom each other, and therefore will also differ from that of thevaporous overhead stream due to the interacting elTects of the overheadstream composition and of the temperature, and pressure followingpartial condensation. Moreover, the compositions of the overheadvaporous product stream and the liquid external reflux stream vary withvariations in accumulator temperature and/or pressure. Nevertheless, itis frequently desirable that the concentration of a key component in thevaporous overhead product be maintained substantially constant at adesired specification.

In accordance with the present invention, it has now been discoveredthat the desired vaporous overhead product specification can bemaintained by maintaining the accumulator pressure substantiallyconstant; establishing a signal representative of the predictedaccumulator temperature, based on measurements of feed flow rate, feedcomposition, overhead product specifications, accumulator pressure, andbottoms product flow rate; comparing the predicted accumulatortemperature with the measured accumulator temperature to obtain a biassignal; and utilizing this bias signal to manipulate the bottoms productflow rate to adjust the accumulator temperature to a value at which theconcentration of a key component of the overhead vaporous product issubstantially equal to the desired concentration thereof.

Accordingly, it is an object of the present invention to provide animproved control system for a partially condensing fractionaldistillation column. It is an object of the invention to provide meansfor maintaining a desired component concentration in a vaporous overheadproduct from a fractional distillation system. Another object of theinvention is to provide a more economical control system having agreater degree of reliability.

Other objects, aspects and advantages of the invention will be apparentfrom a study of the specification, the drawings and the appended claimsto the invention.

In the drawings F l0. 1 is a diagrammatic representation of a fractionaldistillation system embodying a control system in accordance with thepresent invention, and FIG. 2 is a schematic representation of thecomponents of the accumulator temperature computer of FIG. 1.

Referring now to the drawings in detail and FIG. 1 in particular, thereis shown a conventional fractional distillation column 11 which can beprovided with a plurality of vertically spaced liquid-vapor contacttrays. A multicomponent mixture containing a light key component, aheavy key component, at least one lighter-than-light key component, andgenerally at least one hcavier-than-heavy key component, is supplied byway of conduit 12 and introduced onto a feed tray in column 1 1 locatedat an intermediate level therein.

It is customary in multicomponent distillation to designate two of thefeed components as the light key component and the heavy key component.The light key component and the heavy key component are the twocomponents whose recoveries in the overhead product stream and in thebottoms product stream are specified in advance. The light key and theheavy key components may also be considered as the components betweenwhich the split or cut is made. The lighterthan-light key components aremore volatile than the light key component which in turn is morevolatile than the heavy key component. Similarly, the heavy keycomponent is more volatile than the heavier-than-heavy key component.Since perfect separation between the light key component and the heavykey component is impossible, some of the heavy key component will appearas an impurity in the overhead product and some of the light keycomponent will appear as an impurity in the bottoms product. Theoperation of a fractional distillation column can be specified bydesignating the fraction (H of the heavy key component desired in theoverhead product and the fraction (L oi the light key component desiredin the bottoms product. Thus a major portion of the heavy key component,substantially all of the heavier-thanheavy key component, and a minorportion of light key component are withdrawn as a bottoms product streamfrom a lower portion of fractional distillation column 11 by way ofconduit 13. Similarly, a major portion of the light key component,substantially all of the lighter-than-light key component, and a minorportion of the heavy key component are withdrawn from an upper portionof fractional distillation column 11 as an overhead vaporous stream.

This overhead vaporous stream is passed by way of conduit 14 into andthrough condenser 15 to at least partially condense the overheadvaporous stream. The resulting at least partially condensed material ispassed from condenser 15 through conduit 16 into accumulator 17 whereina phase separation occurs. At least a portion of the liquid condensateis withdrawn from accumulator l7 and passed by way of conduit l8, pump19 and conduit 21 into an upper portion of fractional distillationcolumn 11 as external reflux therefor. An overhead vaporous productstream is withdrawn from the vapor space in the upper portion ofaccumulator 17 by way of conduit 22 at a rate controlled by valve 23.

The rate of flow of feed passing through conduit 12 is measured by flowsensor 24 and a physical signal representative of this measurement isapplied to the measurement input of flow controller 25, a first input ofratio relay 26, an input of bottoms flow computer 27, and an input ofaccumulator temperature computer 28. Flow controller 25 manipulatesvalve 29, operatively positioned in conduit 12, responsive to acomparison of the output signal of flow sensor 24 and a setpoint 31representing the desired feed flow rate. Samples of the feed stream arepassed to analyzer 32. Physical signals representa' tive of theconcentration in the feed stream of components of interest are appliedto inputs of computers 27 and 28 and predicted internal reflux to feedratio computer 33. The temperature of the feed stream in conduit 12 ismeasured by temperature sensor 34 and a physical signal representativeof this measurement is applied to inputs of computers 27 and 33. Thetemperature of the external reflux stream in conduit 21 is measured bytemperature sensor 35 and a physical signal representative thereof isapplied to an input of computer 33. Similarly, the temperature of thebottoms product stream in conduit 13 is measured by temperature sensor36 and a physical signal representative thereof is transmitted to aninput of computer 27. A physical signal, H representative of the desiredconcentration of the heavy key component in the vaporous overheadproduct is applied to inputs of computers 27, 28 and 33; while aphysical signal, L representative of the desired concentration of thelight key component in the bottoms product is applied to inputs ofcomputers 27 and 33.

Computer 33 produces a physical output signal, R t/F, representative ofthe predicted ratio of internal reflux to feed in accordance with therelationship:

R =predicted internal reflux flow rate (volume/unit time) F= feed flowrate (volume/unit rate) E =generic symbol for components in feed, eachexpressed as a liquid volume fraction of feed E average column trayefficiency F feed tray (numbering trays from top of column) 1-, feedenthalpy (B.t.u./lb.)

H,, specified liquid volume fraction of heavy key component indistillate product L, specified liquid volume fraction of light keycomponent in bottoms product.

The R t/F signal is applied to a second input of ratio relay 26 toproduce a physical output signal representative of the predictedinternal reflux. This output signal is transmitted to the setpoint inputof flow controller 37.

The temperature of the vaporous overhead stream in conduit 14 ismeasured by temperature sensor 38 and a physical signal representativethereof is applied to an input of actual internal reflux computer 39.The output signal from temperature sensor 35 is applied to another inputof computer 39. The flow rate of external reflux through conduit 21 ismeasured by flow sensor 41 and a physical signal representative thereofis transmitted to an input of computer 39. Computer 39 establishes aphysical output signal R,,,, representative of the actual internalreflux in accordance with the relationship:

R, computed actual internal reflux flow rate (volume/unit time) REMtime) p/ where C, specific heat of external reflux (or liquid on toptray) (B.t.u./unit volumel F.)

A heat of vaporization of liquid on top tray (B.t.u./unit volume) AT=T Twhere T temperature of overhead vapor (or liquid on top tray) Ttemperature of external reflux (F.).

This R,,,, signal is applied to the measurement input of flow controller37. Controller manipulates valve 42, operatively positioned in conduit21, responsive to a comparison of the output signals from computer 39and ratio relay 26.

Computer 27 produces a physical output signal, B, representative of thepredicted bottoms product flow rate in accordance with the relationship:

B predicted flow rate of bottoms product, (volume/unit time) I" genericsymbol for the sum of the light key component and components lighterthan the light key, each expressed as a liquid volume fraction of feed Ffeed flow rate (volume/unit time) Hp specified liquid volume fraction ofheavy key component in overhead product L specified liquid volumefraction of light key component in bottoms product T temperature ofbottoms product at point where bottoms product flow is measured, and

T temperature of feed at point where feed flow is measured.

The output signal B is applied to one input of bias relay, or algebraicadder, 43, the control output of which is connected to the setpointinput of flow controller 44. The flow rate of bottoms product throughconduit 13 is measured by flow sensor 45 and a physical signalrepresentative of the measured flow rate is transmitted to themeasurement input of flow controller 44 and to an input of computer 28.Controller 44 manipulates valve 46 responsive to a comparison of theoutputs of flow sensor 45 and relay 43.

measured external reflux flow rate (volume/unit Pressure sensor 47measures the pressure in accumulator l7 and establishes a physicalsignal representative thereof. This physical signal is applied to themeasurement input of pressure controller 48 and to an input of computer28. A physical signal representative of the desired accumulator pressureis established by setpoint 49 to pressure controller 48. Pressurecontroller 48 establishes at the control output thereof a physicalcontrol signal representative of a comparison of the measuredaccumulator pressure and the desired accumulator pressure. The controlsignal is applied to valve 23 to manipulate valve 23 as required tomaintain the measured accumulator pressure substantially equal to thedesired accumulator pressure.

In order to insure the maintenance of a desired liquid level ofcondensate in accumulator 17, the rate of flow of coolant throughconduit 51 to condenser 15 is varied by valve 52 which in turn ismanipulated by level controller 53 operatively positioned on accumulator17. The rate of flow of the overhead vaporous stream through conduit 14can be controlled by a valve 54 which is manipulated by pressurecontroller 55 responsive to a comparison of the desired overheadpressure represented by setpoint 56 and the actual overhead pressuremeasured by pressure sensor 57. Heat can be supplied to the kettle ofcolumn 11 by circulation of steam or other heat exchange medium fromsupply line 58 into and through reboiler coil 59, the heat exchangemedium being withdrawn by way of conduit 6]. The flow rate of the heatexchange medium in conduit 58 can be controlled by a valve 62, which canbe manipulated by reboiler level controller 60.

Computer 28 produces a physical output signal, T representative of thepredicted accumulator temperature in accordance with the relationship:

wherein T the temperature which would exist for the at least partiallycondensed material at the measured accumulator pressure with a desiredcomposition represented by the desired concentration of the heavy keycomponent and y the presence of substantially all of the at least onelighterthan-light key component contained in the feed stream P measuredaccumulator pressure H desired concentration of said heavy key componentin said vaporous overhead product stream F= flow rate of said feedstream B flow rate of said bottoms product C, concentration in said feedstream of said at least one lighter-than-light key component, and 1) Kconstants. 3) 4) The signal T is applied to the setpoint input oftemperature controller 63. The temperature, T of the partially condensedmaterial in conduit 16 is measured by temperature sensor 64 and aphysical signal representative of the temperature measurement istransmitted to the measurement input of controller 63. The output ofcontroller 63 is representative of a comparison of the predictedaccumulator temperature and the measured accumulator temperature and isapplied to a second input of bias relay 43 as a bias signal for theoutput signal B of computer 27. This results in an adjustment to thebottoms product flow rate. The change in bottoms product flow rate thenaffects the concentration of the heavy key component in the vaporousoverhead stream being withdrawn from the upper portion of column 11. Thechange in the heavy key component concentration results in acorresponding change in the rate at which condensate is produced incondenser 15, thereby affecting the liquid level of condensate in theaccumulator. Liquid level controller 53 then adjusts the coolant flowrate to achieve the desired condensation rate and provide the properaccumulator temperature corresponding to the desired concentration ofsaid heavy key component in the accumulator vapor phase along withsubstantially all of the lighter-than-light key components.

Referring now to FIG. 2, the physical signal C representative of theconcentration in the feed of the lighter-than-light key components, isapplied to one input of multiplier 71. The physical signal P,representative of the feed fiow rate through conduit 12, is applied to asecond input of multiplier 71 and to the minuend input of subtractor 72.The signal 8,, from flow sensor 45 is applied to the subtrahend input ofsubtractor 72. The output of multiplier 71 is applied to the dividendinput of divider 73 while the output of subtractor 73 is transmitted tothe divisor input of divider 73. The output of divider 73 is multipliedby a signal representing K in multiplier 74 and the product istransmitted to an input of algebraic adder 75. The physical signalrepresentative of H is multiplied by a physical signal representing K inmultiplier 76 and the resulting product is applied to an input of adder75. The physical signal representative of P is multiplied by a physicalsignal representing K in multiplier 77 and the resulting product isapplied to an input of adder 75. A physical signal representative of Kis applied to an input of adder 75. The physical signal at the output ofadder 75 is representative of the predicted temperature for accumulator17 for the measured accumulator pressure and with the desiredconcentration of heavy key component in the vaporous overhead product.

While the invention has been illustrated in terms of analog computingand control elements, other types of equipment such as digital computersand direct digital control elements can be employed. The physicalsignals generated by the various elements can be electrical, mechanicalor pneumatic signals or a combination thereof. While it is presentlypreferred that the bottoms fiow computer be utilized to establish thepredicted bottoms flow rate, other means can be employed, including amanual input signal. The use of the accumulator temperature controlsystem of the invention permits a stable control of the composition ofthe vaporous overhead product at a nominal incremental cost in view ofthe low cost of the temperature controller 63 and the components of theaccumulator temperature computer 28.

We claim:

1. In a fractionation system having a fractional distillation column,first conduit means for passing a multicomponent feed stream containinga light key component and a heavy key component and at least onelighter-than-light key component into said fractional distillationcolumn, said at least one lighter-than-light key component being morevolatile than said light key component which in turn is more volatilethan said heavy key component, second conduit means for withdrawing abottoms product from a lower portion of said fractional distillationcolumn, said bottoms product containing at least a major portion of theheavy key component contained in said feed stream, a condenser, anaccumulator, third conduit means for passing an overhead vaporous streamfrom an upper portion of said fractional distillation column into andthrough said condenser to at least partially condense said overheadvaporous stream and for passing the resulting at least partiallycondensed material into said accumulator, said overhead vaporous streamcontaining at least a major portion of the light key component containedin said feed stream and substantially all of said at least onelighter-than-light key component, fourth conduit means for withdrawingcondensate from said accumulator and for passing at least a portion ofthe thus withdrawn condensate into an upper portion of said fractionaldistillation column as external reflux therefor. fifth conduit means forwithdrawing a vaporous overhead product stream from said accumulator;the improvement comprises means for measuring the pressure in saidaccumulator and establishing a first physical signal representativethereof,

means responsive to said first physical signal for controlling thepressure in said accumulator, means for measuring the flow rate of feedpassing through said first conduit means and establishing a secondphysical signal representative thereof, means for analyzing said feedpassing through said first conduit means and establishing a thirdphysical signal representative of the concentration in saidfeed streamof said at least one lighter-than-light key component, means formeasuring the flow rate of said bottoms product through said secondconduit means and establishing a fourth physical signal representativethereof, means for establishing a fifih physical signal representativeof the desired concentration of said heavy key component in saidvaporous overhead product stream, means responsive to said first,second, third, fourth and fifth physical signals for establishing asixth physical signal representative of the temperature which wouldexist for said at least partially condensed material at the measuredaccumulator pressure with a desired composition represented by saiddesired concentration of said heavy key component and by the presence ofsubstantially all of said at least one lighter-than-light key componentcontained in said feed stream, means for measuring the actualtemperature of said at least partially condensed material andestablishing a seventh physical signal representative thereof, meansresponsive to said sixth and seventh physical signals to establish aneighth physical signal representative of a required flow rate of saidbottoms product through said second conduit means, and means responsiveto said eighth physical signal for controlling said flow rate of saidbottoms product through said second conduit means.

2. Apparatus in accordance with claim 1 wherein said means forcontrolling the pressure in said accumulator comprises first valve meansoperatively positioned in said fifth conduit means, a pressurecontroller having a measurement input and a control output, means forapplying said first physical signal to said measurement input, and meansconnected to said control output for manipulating said first valvemeans.

3. Apparatus in accordance with claim 1 wherein said means forestablishing a sixth physical signal comprises means for establishingsaid sixth physical signal in accordance with the relationship wherein Tis the temperature which would exist for said at least partiallycondensed material at the measured accumulator pressure with a desiredcomposition represented by said desired concentration of said heavy keycomponent and by the presence of substantially all of said at least onelighter-than'light key component contained in said feed stream,

P is the measured accumulator pressure,

H1) is the desired concentration of said heavy key component in saidvaporous overhead product stream,

F is the flow rate of said feed stream,

B is the flow rate of said bottoms product,

C is the concentration in said feed stream of said at least onelighter-than-light key component, and

l(,, K K and K are constants.

4. Apparatus in accordance with claim 1 wherein said means for measuringthe actual temperature of said at least partially condensed materialcomprises a temperature sensor operatively connected to a portion ofsaid third conduit means between said condenser and said accumulator.

5. Apparatus in accordance with claim 1 wherein said means forcontrolling said flow rate of said bottoms product through said secondconduit means comprises a second valve means operatively connected insaid second conduit means, and means for manipulating said second valvemeans responsive to said eighth physical signal.

6. Apparatus in accordance with claim 1 further comprising means forcontrolling the degree of cooling effected on said overhead vaporousstream in said condenser responsive to the liquid level of condensate insaid accumulator.

7. Apparatus in accordance with claim 1 wherein said means to establishan eighth physical signal comprises a temperature controller having ameasurement input and a setpoint input and a control output, means forapplying said sixth physical signal to said setpoint input of saidtemperature controller, means for applying said seventh physical signalto said measurement input of said temperature controller, a bias relayhaving first and second inputs and a control output, means forconnecting said control output of said temperature controller to saidfirst input of said bias relay, means for applying to said second inputof said bias relay a ninth physical signal representative of a desiredflow rate of said bottoms product through said second conduit means, thephysical signal at the said control output of said bias relay being saideighth physical signal.

8. Apparatus in accordance with claim 7 further comprising means formeasuring the temperature of said feed stream in said first conduitmeans and establishing a tenth physical signal representative thereof,means for measuring the tem perature of said bottoms product in saidsecond conduit means and establishing an eleventh physical signalrepresentative thereof, means for establishing a twelfth physical signalrepresentative of the desired concentration of said light key componentin said bottoms product, means responsive to said means for analyzingsaid feed stream to establish a thirteenth physical signalrepresentative of the concentration in said feed stream of said lightkey component and said at least one lighter-than-light key component,and means responsive to said second, fifth, tenth, eleventh, twelfth andthirteenth physical signals to generate said ninth physical signal, saidninth physical being representative of a predicted desired flow rate ofsaid bottoms product through said second conduit means,

9. Apparatus in accordance with claim 8 further comprising means forcontrolling the internal reflux in said fractional distillation column.

10. Apparatus in accordance with claim 9 wherein said means forcontrolling the pressure in said accumulator comprises first valve meansoperatively positioned in said fifth conduit means, a pressurecontroller having a measurement input and a control output, means forapplying said first physical signal to said measurement input of saidpressure controller, and means connected to said control output of saidpressure controller for manipulating said first valve means; whereinsaid means for establishing a sixth physical signal comprises means forestablishing said sixth physical signal in accordance with therelationship wherein T is the temperature which would exist for said atleast partially condensed material at the measured accumulator pressurewith a desired composition represented by said desired concentration ofsaid heavy key component and by the presence of substantially all ofsaid at least one lighter-than-light key component contained in saidfeed stream, P is the measured accumulator pressure, H is the desiredconcentration of said heavy key component in said vaporous overheadproduct stream, F is the flow rate of said feed stream, B is the flowrate of said bottoms product, C is the concentration in said feed streamof said at least one lighter-than-light key component, and l(,, K K andK. are constants; wherein said means for measuring the actualtemperature of said at least partially condensed material comprises atemperature sensor operatively connected to a portion of said thirdconduit means between said condenser and said accumulator; wherein saidmeans for controlling said flow rate of said bottoms product throughsaid second conduit means comprises a second valve means operativelyconnected in said second conduit means and means for manipulating saidsecond valve means responsive to said eighth physical signal; andfurther comprising means for controlling the degree of cooling effectedon said overhead vaporous stream in said condenser responsive to theliquid level of condensate in said accumulator.

2. Apparatus in accordance with claim 1 wherein said means for controlling the pressure in said accumulator comprises first valve means operatively positioned in said fifth conduit means, a pressure controller havinG a measurement input and a control output, means for applying said first physical signal to said measurement input, and means connected to said control output for manipulating said first valve means.
 3. Apparatus in accordance with claim 1 wherein said means for establishing a sixth physical signal comprises means for establishing said sixth physical signal in accordance with the relationship wherein TAP is the temperature which would exist for said at least partially condensed material at the measured accumulator pressure with a desired composition represented by said desired concentration of said heavy key component and by the presence of substantially all of said at least one lighter-than-light key component contained in said feed stream, PA is the measured accumulator pressure, HD is the desired concentration of said heavy key component in said vaporous overhead product stream, F is the flow rate of said feed stream, BM is the flow rate of said bottoms product, CF is the concentration in said feed stream of said at least one lighter-than-light key component, and K1, K2, K3 and K4 are constants.
 4. Apparatus in accordance with claim 1 wherein said means for measuring the actual temperature of said at least partially condensed material comprises a temperature sensor operatively connected to a portion of said third conduit means between said condenser and said accumulator.
 5. Apparatus in accordance with claim 1 wherein said means for controlling said flow rate of said bottoms product through said second conduit means comprises a second valve means operatively connected in said second conduit means, and means for manipulating said second valve means responsive to said eighth physical signal.
 6. Apparatus in accordance with claim 1 further comprising means for controlling the degree of cooling effected on said overhead vaporous stream in said condenser responsive to the liquid level of condensate in said accumulator.
 7. Apparatus in accordance with claim 1 wherein said means to establish an eighth physical signal comprises a temperature controller having a measurement input and a setpoint input and a control output, means for applying said sixth physical signal to said setpoint input of said temperature controller, means for applying said seventh physical signal to said measurement input of said temperature controller, a bias relay having first and second inputs and a control output, means for connecting said control output of said temperature controller to said first input of said bias relay, means for applying to said second input of said bias relay a ninth physical signal representative of a desired flow rate of said bottoms product through said second conduit means, the physical signal at the said control output of said bias relay being said eighth physical signal.
 8. Apparatus in accordance with claim 7 further comprising means for measuring the temperature of said feed stream in said first conduit means and establishing a tenth physical signal representative thereof, means for measuring the temperature of said bottoms product in said second conduit means and establishing an eleventh physical signal representative thereof, means for establishing a twelfth physical signal representative of the desired concentration of said light key component in said bottoms product, means responsive to said means for analyzing said feed stream to establish a thirteenth physical signal representative of the concentration in said feed stream of said light key component and said at least one lighter-than-light key component, and means responsive to said second, fifth, tenth, eleventh, twelfth and thirteenth physical signals to generate said ninth physical signal, said ninth physical being representative of a predicted desired flow rate of said bottoms product through said second conduit means.
 9. Apparatus in accordance with claim 8 further comprising means for controlling the internal reflux in said fractional distillation column.
 10. Apparatus in accordance with claim 9 wherein said means for controlling the pressure in said accumulator comprises first valve means operatively positioned in said fifth conduit means, a pressure controller having a measurement input and a control output, means for applying said first physical signal to said measurement input of said pressure controller, and means connected to said control output of said pressure controller for manipulating said first valve means; wherein said means for establishing a sixth physical signal comprises means for establishing said sixth physical signal in accordance with the relationship wherein TAP is the temperature which would exist for said at least partially condensed material at the measured accumulator pressure with a desired composition represented by said desired concentration of said heavy key component and by the presence of substantially all of said at least one lighter-than-light key component contained in said feed stream, PA is the measured accumulator pressure, HD is the desired concentration of said heavy key component in said vaporous overhead product stream, F is the flow rate of said feed stream, BM is the flow rate of said bottoms product, CF is the concentration in said feed stream of said at least one lighter-than-light key component, and K1, K2, K3 and K4 are constants; wherein said means for measuring the actual temperature of said at least partially condensed material comprises a temperature sensor operatively connected to a portion of said third conduit means between said condenser and said accumulator; wherein said means for controlling said flow rate of said bottoms product through said second conduit means comprises a second valve means operatively connected in said second conduit means and means for manipulating said second valve means responsive to said eighth physical signal; and further comprising means for controlling the degree of cooling effected on said overhead vaporous stream in said condenser responsive to the liquid level of condensate in said accumulator. 